Vibration detectors, or limit level switches, for detecting or monitoring fill level, phase boundary or density of a medium in a container are already known. They are available from Endress+Hauser in many embodiments under the names LIQUIPHANT and SOLIPHANT.
Vibration detectors usually have an oscillatory rod or an oscillatory fork as the oscillatory element; the oscillatory element is secured to a membrane or diaphragm. Moreover, so called membrane oscillators, in which the additional oscillatory element is omitted, are also known. The membrane is clamped in a sensor housing and is excited to oscillations by an electromechanical or a piezoelectric drive. A stack, or bimorph, drive is usually used as transmitting/receiving unit. The excitation of the oscillatable unit occurs at the resonance frequency—thus here a so called fundamental wave excitation—wherein the phase shift between the transmission signal and the received signal usually lies in a defined range. Usually, this range lies between 46° and 140°.
Vibration detectors embodied as fill level measuring devices utilize the effect that the oscillation frequency and the oscillation amplitude depend on the respective degree of covering of the oscillatory element: While the oscillatory element can execute its oscillations freely and undamped in air, as soon as it becomes immersed partially or completely in the medium, the oscillatory element experiences a frequency and amplitude change. On the basis of a predetermined frequency change, the reaching of the predetermined fill level or a predetermined phase boundary level of the medium in the container can thus be unequivocally deduced. Fill level measuring devices are, moreover, used principally as overfill protection or for the purpose of protection against a pump running empty.
Moreover, the oscillation of the oscillatory element is also influenced by the respective density of the medium, since the mass moved changes with density. Therefore, with a constant degree of covering, a functional relationship with the density of the medium exists, so that vibration detectors are well suited both for fill level determination as well as for density determination.
In practice, the oscillations of the membrane are recorded for the purpose of monitoring and detecting the fill level or the density of the medium in the container and are converted to electrical, received signals by means of at least one piezoelectric element. The electrical received signals are then evaluated by evaluating electronics. In the case of fill level determination, the evaluating electronics monitors the oscillation frequency and/or the oscillation amplitude of the oscillatory element and signals the state ‘sensor covered’ or ‘sensor uncovered’ as soon as the measured values drop under or exceed a predetermined reference value. A corresponding report to operating personnel can occur via an optical and/or an acoustic path. Alternatively or supplementally, a switching operation is triggered; so, for instance, an inlet valve or a drain valve on the container is opened or closed.
The devices for measuring fill level, phase boundary level or density mentioned above are applied in many branches of industry, for example, in the chemicals industry, in the foods industry or in water processing. The range of fill substances monitored extends from water to yogurt, paints and lacquers, to high viscosity media such as honey, and to strongly foaming media, such as beer.
Known vibration detectors are distinguished by a closed oscillatory circuit constructed of analog components. In order to be able to operate the different types of vibration detectors, the analog components must be matched to the respective embodiments of the vibration detectors as optimally as possible. Among other things, the particular geometry of the oscillatable unit plays a large role. Furthermore, a microcontroller is provided, which performs the evaluation of the ascertained frequency and controls communication with a central computer.
In addition to high development time and monetary costs, the individual solutions previously mentioned lack the desired high flexibility and a compact form of construction. Thus, known vibration sensors often only permit a statement about whether the oscillatable element is covered or not. Also, special electronic components often come into use for the control/evaluation unit, which is disadvantageous, since these components have relatively strict component tolerances. It is also disadvantageous that there is no protection against reverse engineering in the case of known detectors comprising electronic components: A competitor can replicate a vibration detector with knowledge of the components without possessing detailed knowledge of the functional principle of the detector.